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1 In Situ Chemical Reduction using Zero valent Iron injection A technique for the remediation of source zones

2 Summary Zerovalent iron (ZVI) can be used for the remediation of soil and groundwater contamination with chlorinated solvents. ZVI has the ability to dehalogenate chlorinated compounds by chemical reduction and has been used since many years as a granular material in permeable reactive barriers for the treatment of contaminated groundwater. Due to the high specific surface area, nano- and micro- sized ZVI-particles (nzvi & mzvi) are more reactive than granular materials. Moreover, nzvi- and mzvi-particles can be readily injected into the soilmatrix, thus allowing for active treatment of source and plume areas, even at greater depth. Nowadays there are many different types of ZVI-materials available on the market. ZVI-particles can be differentiated based on size (nano (<1µm) or micro (> 1µm)) and on constitution: Catalysed bimetallic ZVI-particles consist of zerovalent iron/metal and a catalyst (Pt, Pd,..) thus generating more reactive materials which cause higher degradation rates. Supported ZVI-particles consist of ZVI-particles attached on a non-metallic carrier material which mostly serves to create higher stability and mobility of the ZVI suspension. Emulsified ZVI-particles (EZVI) are developed to directly treat the free phase of chlorinated solvents (DNAPL). The ZVI-particles are surrounded by a biodegradable oil-based hydrophobic membrane. Since the remediation with injectable ZVI-particles is based on direct contact between the ZVI-particle and the contaminant, the mobility and stability of the ZVI-particle in the soil is of crucial importance for the effectiveness of the remediation. Based on mathematical models and previous experiments, the mobility of non-modified nzvi-particles in the soil is limited to a few centimetres. The limited mobility is mainly due to aggregation of ZVI-particles (electromagnetic forces), ZVI-soil particle interactions and geochemical conditions. Mobility of ZVI-particles can be increased by Surface modifications of ZVI-particles to prevent aggregation; The implementation of high injection velocities; Mechanical modifications of the subsurface via fracturing (pneumatic or hydraulic) or dilatation (pressure pulse technology). A thorough preliminary study is necessary to check the feasibility of ZVI-particle injection (ISCR) for the treatment of chlorinated solvents. The preliminary study consists of the following phases: 1. Contaminated site characterisation (conceptual site model (CSM)) The contaminated soil volume and depth is essential to determine the required injection depths, distances and volumes. Information about the amounts of electron donors, contaminants and other electron acceptors (nitrate, sulphate, oxygen, Fe(II)/Fe(III) ) is essential to determine the needed amount of ZVI. Hydrogeological parameters such as hydraulic permeability, the average groundwater flow velocity and groundwater flow direction are needed to determine the radius of influence of the injections, the volumes that can be injected, the time period of injection, reflux of the injected solution and the number of injections (distance between injection points). In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 2

3 2. Lab tests Lab tests allow for the investigation of degradation kinetics (batch tests), stoichiometry (batch test with aquifer samples), potential to inject a ZVI-solution, mobility (column tests), stability of the ZVI-particle suspension (sedimentation tests) and overall feasibility of a ZVI application for a particular site. 3. Field test Since it is difficult to exactly simulate the conditions in the aquifer, it is recommended to conduct a field test. A field test can i.a. provide information about the injection method and the maximum injection pressure, flow rate and radius of influence. It also allows for the observation of possible rebound effects and the establishment of a reasonable remediation target. ZVI-particles can be injected via several injection methods. The chosen injection method is of great influence on the rate of influence of the injection. Each injection method has is own specific advantages and limitations and the choice is, amongst others, determined by the site specific conditions and available remediation budget. During injection it is important to avoid contact between the ZVI and oxidizing agents since these diminish the reactivity and, in addition, can cause safety hazards due to strong exothermic reactions. ZVI-particle injection is an expensive remediation technique since ZVI-particles (especially the nano-sized and/or modified ZVI-particles) are expensive and the radius of influence (because of limited mobility) is low. Based on literature and experience, ZVI-particle injection for the remediation of CVOC contaminated aquifers is best used in combination with an injection of substrate to enhance the natural degradation of CVOC s in order to achieve a (cost) effective remediation. The combined injection of ZVI and organic substrate will manipulate geochemical conditions in order to optimize both abiotic and biotic degradation of CVOC-contaminations by creating e.g. optimal ORP (oxidoreduction potential) and DO (dissolved oxygen) conditions in the subsoil. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 3

4 Table of content Summary... 2 Table of content Introduction CityChlor and the integrated approach CityChlor and technical innovations Contents and structure Content composition Discussion of the technique Structure of the study Glossary List of abbreviations Characteristics of nano- and micro-scale (bi)metallic particles Background Micro- and nano-scale zero-valent iron Micro-scale zero-valent iron (mzvi) Nano-scale zero-valent iron (nzvi) Modified nzvi Production and availability of NZVI particles Group 1: Production of nano-particles from individual atoms (bottom-up) Group 2: Production of nano-particles through the refinement of rougher materials (top-down) Commercially available nano-scale & micro-scale iron Reaction mechanisms Transport and mobility of mzvi and nzvi particle Factors that influence the mobility of mzvi and nzvi particles Surface modifications of MZVI and NZVI particles Known possibilities and limitations of ZVI particles Known possibilities Known limitations Practice: application of ZVI particles for the treatment of soil contamination Introduction Practical application Preliminary study Introduction Characterisation of the site to be remediated In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 4

6 1 Introduction 1.1 CityChlor and the integrated approach Space is scarce in Europe. Even in the subsurface it is getting busier. Large-scale soil and groundwater contamination with chlorinated solvents are often an obstruction for urban developments. The traditional way of dealing with polluted soil and groundwater does not work in all cases and is not economically and sustainable feasible. In urban environments multiple contaminations with chlorinated solvents are often mixed with each other and spread underneath buildings. This not only leads to technical problems for remediation, but also to liability and financial discussions and hence has an impact on society. An integrated approach and area-oriented approach is needed to tackle the problems. The CityChlor project has demonstrated that remediation and sustainable development can evolve on a parallel timescale. An integrated approach combines all aspects that are relevant to tackle the problems that pollution with VOC in urban environment causes. Depending on area, site and context different aspects together or parallel to each other can be used. Not only technical solutions are included, but also socio-economic aspects as urban development, communication, financial and legal aspects, time, space, environment and actors (active & passive) have to be handled. CityChlor did not remain at single case remediation, but looked at the area as a whole in a bigger context: the area-oriented approach. A technical approach that makes it possible to remediate, monitor and control multiple groundwater sources and plumes within a fixed area. 1.2 CityChlor and technical innovations The managing of knowledge and technical innovations are one of the key to achieve a sustainable city development. A development project has to cope with loads of information coming from different disciplines in different (technical) languages and with different uncertainties. With chlorinated solvents, the knowledge about the pollution will always have a certain uncertainty that can have an impact on the course and the costs of the remediation. An efficient 'managing of knowledge' will try to decrease this degree of uncertainty. CityChlor therefore also worked on the technical aspects of characterization and remediation. The conventional techniques that are applied for investigation and remediation have their limitations dealing with chlorinated solvents. Promising innovative techniques exist, but do not easily find their way to current application. This barrier is often caused by lack of knowledge on different levels. Experts and contractors do not always have the means to invest in experiments with new techniques, authorities are reluctant to accept techniques of which the results may be uncertain and clients aren't eager to pay for experimental techniques. Dissemination of knowledge can break this deadlock. CityChlor therefore collected experiences from field application of innovative techniques and implemented itself a number of techniques in pilot projects. For the detailed outcomes, the reader is referred to the specific reports. CityChlor - new solutions for complex pollutions In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 6

7 2 Contents and structure The document before you contains a technical summary of the state of play with regard to the use of injections with zero-valent (nano or micro-scale) iron for treating ground and groundwater contamination with chlorinated solvents. Given the fact that practical experience within the (European) market with respect to this technique was rather limited when the document was written there was opted on elaborating a technical summary instead of a code of best practice. This document, however, not only contains a summary of the technological state of play, but will also enable the reader to evaluate the practicality of this technique and also provide a guideline for the actual usage thereof. 2.1 Content composition The content of this document has been composed on the basis of 1. Experiences from the CityChlor pilot test in Herk-de-Stad 2. Literature study (see bibliography) The literature study by Leen Bastiaens (VITO) Injection of (bi)metallic nano-scale iron particles into aquifers contaminated with chlorinated hydrocarbons, Phase 1 was the basis for this literature study. Additional information was obtained from the following sources: Background information gathered by OVAM, including, among others, articles from U.S. EPA (with Superfund sites, among others), Aquarehab, Nanofrezes Scientific articles from U.S. EPA Scientific articles & proceedings from the 2010 Consoil conference Scientific articles & proceedings from the 2006 & 2010 Batelle conferences Information available via the Internet 3. Survey of soil remediation companies and suppliers (see chapter 7) 2.2 Discussion of the technique Over the past few years a lot of research has been conducted on new or improved remediation techniques for the treatment of soil and groundwater contamination with volatile organic chlorine compounds (VOCl). As soil contaminated with VOCl is mostly present in urbanised areas and, due to the nature of the contaminants, is often not easily accessible for conventional remediation techniques, such as excavation, there is increasing demand for in situ remediation techniques capable of remediating this contamination in its different phases (dissolved, free phase), at a great depth and in an efficient and effective manner. Until the early 1990s pump & treat was the most common remediation technique for the treatment of groundwater contaminated with VOCl. As this technique in many cases is not effective (due to low solvability of VOCl) and sustainable, cause long-term costs and can even be very expensive and slow (the average pump & treat system in the US is active for 18 years (U.S. EPA, 2001)), this technology was in several cases In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 7

8 replaced with passive in situ remediation techniques, such as (bi)metallic permeable barriers ( permeable reactive barrier : PRB). PRBs have been applied in the field for more than 10 years (Matheson & Tratnyek; 1994; Gillham, 1996; Gavaskar, 1997; O Hannesin & Gillham, 1998; Bastiaens et al., 2002a, 2002b; Dries et al., 2004). Due to the following limitations of permeable reactive barriers (Watlington, 2005): Can only be used for treatment of the dissolved phase (~plume remediation), this means this is more or less a safety measure for the downstream aquifer Less reactive for lightly chlorinated products (components with Cl content < trichloro ethene) Can be applied down to a maximum depth of approx. 15 m bgl (this is a question of the construction technology you use) Relatively high investment cost with a replacement period which is hard to estimate (gradual reduction of reactivity due to formation of iron hydroxide and iron carbonate deposition) there is a growing interest in other in situ remediation techniques, such as: Thermal treatment In situ chemical oxidation (ISCO) In situ chemical reduction (ISCR) Surfactant co-solvent flushing Stimulation of natural attenuation (ENA) This literature study focuses on in situ chemical reduction (ISCR) through the use of injectable micro- and nano-scale zero-valent (bi)metallic particles for the treatment of soil contamination with chlorinated hydrocarbons. 2.3 Structure of the study 1. Characteristics of nano- and micro-scale (bi)metallic particles In this chapter an overview is given of the commercially available nano- and micro-scale (bi)metallic particles and their properties. 2. Known possibilities and limitations of iron injections In this chapter an overview is given of the known applications of ZVI-particles and their limitations. 3. Practice: applicability of iron particles for the treatment of soil contamination with VOCl In this chapter the practical aspects of this remediation technique will be looked at: 1. Set-up of laboratory and pilot tests 2. Application methods 3. Follow-up of the remediation 4. Cost of iron injection In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 8

9 This chapter gives an idea of the cost of iron particles and the cost of remediation per m³ of treated soil, based on the literature studied. 5. Results of market survey This chapter explains the way of market survey with the results. 6. Conclusions 7. Cases 2.4 Glossary Adsorption Anionic Soil Colloid Desorption DNAPL Electron acceptor Electron donor Electrostatic stabilisation Emulsion Process whereby a substance adheres via physical-chemical forces to the surface of a solid. Negatively charged particles (ion). The solid constituents of the earth, also including the groundwater and other components and organisms that form part of it or live therein. A small particle (or collection of particles) that is larger than one molecule and has a diameter of between 1 and 1000 nm. Process whereby adsorbed compounds are released in the water phase. Layer of organic (non-aqueous) liquids with a greater density than water. See oxidants. See reductants. Stabilisation of particles via use of electrorepulsion (or mutual repulsion from particles with the same electrical charge). An emulsion is a mixture of two immiscible liquids that, under normal circumstances, do not form a stable or homogenous mixture. To obtain an emulsion, a surfactant is required in order to create a stable mixture. An emulsion typically forms a colloidal mixture. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 9

10 Geogenic Catalyst Non-saturated area Solubility Oxidant Permeability ph PRB Reagent Redox potential Redox reaction Reductant Steric stabilisation Concentration that originates in geological formation and is thus natural in this formation. A substance that affects the reaction speed of a chemical reaction without being used up in the chemical reaction itself. Zone in de soil above the groundwater table. A measure that expresses the maximum amount of a product that can be dissolved in water without forming precipitation or a LNAPL or DNAPL. Chemical substance with a high redox potential. The oxidant acts as an electron acceptor in the redox reaction. The valency state of the reductant decreases in the redox reaction. A measure of the speed at which a fluid can pass through a permeable medium. Acidity Permeable reactive barrier. Artificial barrier in the subsurface that is used as a management tool for groundwater pollution. The dissolved pollution is typically broken down in the barrier via oxidative or reductive reactions. A chemical substance that reacts in a chemical reaction. Unit used to express the degree to which electrons are available for redox reactions. Chemical reaction whereby electrons are transferred from the electron donor to the electron acceptor. Chemical substance with a low redox potential. The reductant acts as an electron donor in the redox reaction. The valency state of the reductant increases in the redox reaction. Stabilisation of particles via the adherence of molecules that, as a result of the volume that they take up, inhibit the advance and aggregation of the particles. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 10

12 3 Characteristics of nano- and microscale (bi)metallic particles 3.1 Background The use of zero-valent metals for environmental applications was first described in the literature in 1972 (Sweeny & Fischer, 1972). Years later, the observed disappearance of trichloroethene (TCE) in metal probes was a starting point for a more detailed study on the use of zero-valent metals (mainly Fe(0)) for the remediation of groundwater contaminated with VOCl. (Gillham & O Hannesin, 1994; Gillham et al., 1998; Tratnyek et al., 2003). Granular (bi)metallic materials (> 50 μm), such as, for instance, granular zero-valent iron (ZVI), have been studied for years for the in situ removal of VOCl from groundwater in PRBs. (Bastiaans et al.) For a few years now there has been a special interest in finer (bi)metallic materials, such as micro-scale (Cantrell & Kaplan, 1997; Choe et al. 2000) and nano-scale (bi)metallic particles (Wang & Zhang, 1997; Ponder et al., 2000; Lien & Zhang, 1999, 2001; Li et al., 2003; Zhang, 2004). Due to their high specific surface, these materials are more reactive than granular materials, and due to their small particle size they can be used in more diverse applications. The high reactivity of these materials makes it possible to remove a wide range of pollutants from the groundwater (Reference????). Moreover, it is possible to apply the material via injection at a great depth. Modified nano-scale zero-valent iron (nzvi), concretely emulsified nano-scale zero-valent iron (EZVI), even makes it possible, in theory, to treat free phase (DNAPL). The injection of nanoscale and micro-scale zero-valent iron (mzvi) constitutes the basis of a new generation of in situ remediation techniques for the treatment of soil contamination with VOCl. 3.2 Micro- and nano-scale zero-valent iron Fine ZVI materials can be subdivided into different categories based on particle size. An additional subdivision can be made based on the composition of the particles (metallic, bimetallic), surface modifications and the solution medium or carrier material (supported, emulsion, ) Micro-scale zero-valent iron (mzvi) Micro-scale (bi)metallic particles are, strictly speaking, particles with a diameter greater than 1µm Nano-scale zero-valent iron (nzvi) Nano-scale zero-valent iron particles are, strictly speaking, (bi)metallic particles with a diameter smaller than 100 nm. In certain cases, colloïdal particles (1-2μm) are also incorrectly included with nzvi. In this report, nzvi comprises both metallic and (bi)metallic nano-scale particles. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 12

13 nzvi consists of a mixture of zero-valent iron and iron oxides. More concretely, the following two variants can be distinguished: Pure nano-scale zero-valent iron (nzvi) nzvi particles mainly consist of zero-valent iron (80-90% wt%) and, depending on the producer, have the following specifications: Particle diameter nm Specific surface area m²/g Reactive nano-scale iron product (RNIP) Reactive nano-scale iron product consists of a mixture of 50/50 wt% zero-valent iron and magnetite (Fe 3O 4), in which the magnetite forms the outer shell of the particle. In a study by Okinaka et al. (2004), the average particle size is approx. 70 nm and the average specific surface area 28.8 m²/g. The larger specific surface area and the higher zero-valent iron content of nzvi are responsible for a higher degradation rate of VOCl (~dechlorination) than that observed for RNIP. However, the outer magnetite shell of RNIP slow down reaction with water (H 2 formation), as a result of which the life of such particles is extended considerably (Liu et al., 2005). The limited stability and mobility (see paragraph 3.6 and 3.7) of pure nzvi provides the impetus for the development of modified nzvi Modified nzvi A. Catalysed bimetallic nano-scale particles Besides the aforementioned classic particles, there is a growing offer of nano-scale bimetallic particles which, in addition to zero-valent iron or another metal, contain a second metallic material. Here, one metal (Fe, Zn, ) is mainly the electron donor, and the other (Pd, Pt, Ni ) the catalyst in the reaction. Such nanoscale bimetallic particles are also sometimes called catalysed nzvi particles. With catalysed nzvi particles much higher reaction rates can be obtained, with the disadvantage that the life of such particles is limited. Examples of such materials are: Fe/Pd, Fe/Ag, Fe/Ni, Fe/Co, Fe/Cu, Zn/Pd (Zhang et al., 2003) Ag/Pd, Au/Pd (Nutt et al, 2005) Fe/Pd particles cause high reaction rates 1 in comparison with pure nzvi. In addition, Pd has a high selectivity for the C-Cl bond. However, research by Huang et al. (2009) shows that Ni is a cheaper alternative for the expensive Pd. Fe/Ni particles can achieve the complete degradation of perchloroethene (PCE) to ethane. Catalysed nzvi is mostly used in the US (40% of nzvi remediations), but has not yet been used in Europe (data for 2010). In Europe, the opinion is held that the possible advantages of catalysed nzvi particles do not 1 Pd and Ni are hydrogenation catalysts which play an important role in the transfer of H2. Moreover, Pd and Ni as transition metals have free electron shells which make it possible to lower the activation energy via transition bonds with the p-electron pair or -bond of the chlorine atom in chlorinated organic components (Huang et al., 2009). In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 13

14 compensate for the possible disadvantages (higher costs, short life and possible toxicity of the catalyst) (Müller, Nowack, 2010). B. Supported nano-scale particles Supported nano-scale particles are nzvi particles that are attached to a non-metallic carrier. Ponder et al. (2000) have conducted research on carbon nano-particles onto which nzvi particles were attached. The pellets obtained have a diameter of 50 to 200 nm and are made anionic before they are introduced into the soil. The anionic carrier material is supposed to impede the aggregation and sedimentation of the particles, as a result of which the injectability, the mobility and hence the spread of the particle are increased. Supported nano-scale particles with a hydrophilic carrier consisting of polymers, e.g. poly(acrylic acid), have also been described (Ponder et al., 2001, Schrick et al., 2004). C. Emulsified nzvi particles (EZVI) EZVI has been specifically developed for the treatment of free phases of chlorinated solvents (DNAPLs, Dense Non-Aqueous Phase Liquids) (Quinn et al., 2003). Plant oils and all kinds of surfactants can be used to prepare this emulsion. More specifically, this is a surfactant-stabilised biodegradable emulsion which forms drops with an oil-water membrane around the nzvi or mzvi particles in water drops. Emulsions typically have a diameter of approx. 40 µm and a specific gravity of approx. 1.1 kg/l (Quinn 2005, O Hara 2004). As the emulsion also behaves as a DNAPL, it will move through the soil in the same way as the DNAPL to be remediated, thus allowing for maximum contact. A graphic representation of this emulsion can be found in Figure 1. Figure 1: Graphic representation of emulsified NZVI particles (left) & microscopic view of emulsified NZVI particles (right) The following hypothetic reaction mechanism is proposed: DNAPL components diffuse from the oil layer to the enclosed water phase, where they react with the zero-valent iron. Generated degradation products diffuse with increasing concentrations from the enclosed water phase through the oil layer to the surrounding water. The oil membrane around the nzvi particles offers protection against premature oxidation by oxygen and other components (e.g. inorganic material, such as sulphate and nitrate). The reactivity of classic nzvi In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 14

15 particles and EZVI particles is comparable (Quinn et al., 2005). In the longer term, the oil phase can serve as an electron donor in the biological degradation of residual VOCl concentrations. 3.3 Production and availability of NZVI particles Diverse methods for the production of NZVIs have been described in the literature and can be subdivided into 2 groups Group 1: Production of nano-particles from individual atoms (bottom-up) Wang and Zhang (1997) synthesise nano-particles via chemical reduction of Fe 3+ by sodium borohydride. More specifically, 0.2 M NaBH 4 is added to FeCl 3.2H 2O (0.05 M), which results in nanoparticles with a diameter between 1 nm and 100 nm, an average size of 50 ± 15 nm and a specific surface area of m²/g. The method allows for large amounts to be produced, but it is expensive, up to 5,000 $ per kg (Vance et al., 2002). It is also possible to heat iron pentacarbonyl to C. At this temperature, nzvi and carbon monoxide are produced. This method allows to obtain nano-particles with a diameter of 5 nm. (bi)metallic materials are produced via reductive precipitation (Zhang et al., 2003). For instance, palladium-coated nano-scale iron particles can be obtained by bringing freshly made nano-scale iron particles into contact with palladium acetate (1 %W). As a result of reduction, a Pd layer will form on the iron material (Wang & Zhang, 1997). Other (bi)metallic materials can be obtained in a similar manner (Xu & Zhang, 2000). With the micro-emulsion method nano-particles are produced via chemical reduction with sodium borohydride. However, these reactions take place in small aqueous environments in an oil phase, which allows for the production of nano-materials with a controlled size and shape (Li et al., 2003). The materials obtained have an average diameter smaller than 10 nm. Nano-iron particles can also be produced through reduction of goethite by means of heat (up to 600 C) and hydrogen gas. In this process, iron sulphate, sodium carbonate and sodium hydroxide are used as base materials to make goethite precursors. By dehydration of these goethite precursors, hematite precursors are then produced, which are further reduced with hydrogen gas to nano-iron (Uegami et al, 2003). The sol-gel method was mentioned by Li et al. (2003). A gel containing silicon tetraethoxide and a metal component, such as FeCl 3, is heated. A glass-metal nano-composite is produced which contains ultra-fine iron particles. Aside from these methods there is also electrolysis (Fisher). In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 15

16 Toda RNIP is a crystalline nano-iron which is produced by a reduction of FeOOH in the gas phase. This reaction produces particles with an average size of 70 nm and a specific surface area of 29 m²/g. These specific nano-particles are coated with polyacrylic acid. NANO IRON produces nzvi particles from nano-particles of ferrihydrite ((Fe 3+ )2O 3 0.5H 2O) Group 2: Production of nano-particles through the refinement of rougher materials (top-down) Vance and his co-authors (2002) use a ball mill system to refine a powder of an elemental metal (1-10 μm) to a nano-material ( nm). During this process a non-aqueous organic liquid and a dispersant are added. Via this method materials are produced in amounts up to 10 kg. The production of larger amounts would be possible (NAPASAN Project > 300 kg). 3.4 Commercially available nano-scale & micro-scale iron The number of producers of NZVI & MZVI and, concretely, those able to deliver large amounts, are currently still relatively limited. 1. NANO IRON (Czech Republic) Czech company specialised in the production of nzvi particles. The particles have an average size of 50 nm (between 20 and 100 nm), a specific surface area of m²/g and a high zero-valent iron content of approx. 80/90 wt%. The cost depends on the amount and nature of the product and varies between Euro/kg for the nzvi slurry (20% dm). The following products are commercially available: nzvi powder (100 g, 1 & 5 kg) NANOFER STAR: Surface-modified iron. Stable in normal atmospheric conditions NANOFER 25P: Unmodified iron. Only stable in inert (nitrogen) atmosphere nzvi slurry (10, 20 & 40 kg, 20% dm) NANOFER 25S: slurry with organic and inorganic stabilisers NANOFER 25: slurry with only inorganic stabilisers 2. TODA KOGYO CORP. (Japan) TODA KOGYO is a Japanese company specialised in the production of nano-scale RNIP particles (mainly iron oxides). The products are offered as a powder and as a slurry. The particles have an average size of 100 nm and a specific surface area of approx. 23 m²/g. The cost depends on the amount and varies between Euro/kg (powder). ANIP-10DS (Active Nanoscale Iron Particles): metallic ANIP-20DS: metallic, finer material than ANIP-10DS, not commercially available; 3. Polyflon company (US, Florida) In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 16

17 Polyflon company produces nzvi particles. The particles have a size between nm and a specific surface area between 37 and 58 m²/g. The cost is unknown. PolyMetallixTM Particles 4. PARS Environmental inc. (US, NJ) PARS is an American environmental consultancy firm that has developed its own nzvi particles. Product specifications are unknown and it is also unclear whether the nzvi particles are available commercially. NanoFeTM: nano-scale zero-valent iron particles NanoFe PlusTM: a modified NanoFeTM with enclosure of a catalyst and an additive to boost the speed and efficiency of the remediation. (Varadhi et al., 2005a); 5. TOYO INK MFG. CO. LTD (Japan) Toyo Ink is a company specialised in the production of ink, paint, colouring agents and hence colloidal dispersions, including dispersions of colloidal micro-scale iron (50% average diameter: 2080 nm). 6. Gotthart Maier Metallpulver GmbH GMM produces metal powders with, among other things, micro-scale iron powder. mzvi particles from GMM were used for the pilot test in Herk-de-Stad. The product used has a size between 0-80 µm, a zero-valent iron content of approx. 90 wt% and contains traces of other metals. The material is available at a cost of 1.2 euro/kg 7. Lehigh University (US, Pennsylvania) No permanent product, no commercial production 8. Golder 9. Aventus 3.5 Reaction mechanisms Zero-valent iron is capable of reducing chlorinated compounds to harmless components, as shown in the general reaction formula below: 2 Fe 0 + R-Cl + 3 H 2O 2 Fe 2+ + R-H + 3 OH - + H 2 + Cl - (R = Aryl group) The chlorinated compounds can be reduced as follows: 1. Reduction at the metal surface in the presence of a proton donor Fe 0 + R-Cl + H + Fe 2+ + R-H + Cl - (1) 2. Continuation of reaction 1 by the further oxidation of Fe 2+ to Fe 3+ In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 17

19 Figure 2: Possible reaction mechanisms for reductive dechlorination of VOCls with zero-valent iron According to reaction formulas (3) and (5) the reactions with zero-valent iron cause an increase in ph (2 to 3 units in laboratory conditions (Zhang, 2003)) and a reduction in redox potential (500 to 900 mv reduction in laboratory conditions (Zhang, 2003)). It is expected that in practice the increase in ph and the reduction in redox potential will be less spectacular due to the buffering capacity of the ground water and the decrease in the chemical reactions due to diffusion and dispersion. The influence of zero-valent iron on the geochemical environment (consumption of oxygen, nitrate, sulphate and production of hydrogen & Fe (II)) of the soil can lead to the stimulation of the growth of anaerobic microorganisms and can therefore contribute to an accelerated natural reductive degradation of chlorinated components. The degradation of chlorinated ethenes can take place via two degradation paths, which have been summarised in Figure 4 for PCE. The hydrogenolysis degradation path results in the formation of the undesired degradation products cis-dichloroethylene and vinyl chloride. The ß-elimination degradation path takes place via the harmless acetylene. The extent to which each of the degradation paths is followed depends on the type of iron. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 19

21 Trihalomethanes Pentachlorophenol Bromoform Other organic components Dibromochloromethane N-nitrosodimethylamine Dichlorobromomethane TNT Heavy metals Inorganic components Mercury Dichromate Nickel Arsenic Silver Perchlorate Cadmium Nitrate Table 1: Components which are degradable with zero-valent iron 3.6 Transport and mobility of mzvi and nzvi particle Factors that influence the mobility of mzvi and nzvi particles As the remediation technique with zero-valent iron is based on direct contact between the surface of the ZVI particle and the dissolved contaminant, the mobility of the ZVI particles is of crucial importance for the effectiveness of the remediation. According to Lowry (2005), the transport of unmodified nzvi is limited to a maximum of a few metres. Experiments at the university of Stuttgart have even shown that the transport of unmodified iron is limited to a few cm. Based on arithmetic models, the mobility of nzvi particles is estimated to be limited, unless the following points are applied: 1. Adjustment of the nzvi particles by means of surface modifications, so that aggregation is prevented as much as possible 2. High injection speeds in comparison with the natural groundwater flow 3. Mechanical adjustments to the soil in the form of cracks (pneumatic & hydraulic fracturing) and/or dilation of the soil particles (pressure pulse technology) Figure 4: Attachment of NZVI particles to soil particles leading to blocked soil pores The mobility and transport speed of nzvi particles in the soil is mainly influenced by the following 4 factors: 1. Aggregation and potential 2. nzvi-soil particle interactions 3. Geochemistry 4. Application method 5. Permeability of the aquifer 6. Particle size In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 21

22 1. Aggregation and potential The aggregation of ZVI particles causes a reduced mobility and reactivity. The reduction in mobility is caused by the blockage of the soil pores (see Figure 4) and by the attachment to soil particles. The reduction in reactivity is the result of the reduction in the specific surface area and the blockage of reactive sites on the particle. The aggregation of ZVI particles is caused by the following processes: Concentration of ZVI particles in solution: Recent research has shown that the particles are mobile at low concentrations (e.g. 30 mg/l) and that this does not depend on particle diameter distribution and magnetic forces (Phenrat et al., 2009b). Magnetic forces: large particles with a higher Fe(0) content have a stronger magnetic field and will preferably aggregate with other ZVI particles. Small particles with a lower ZVI content are less susceptible to aggregation and hence more mobile. As the ZVI content is essential for the degradation potential of the particle, the increased mobility and the increased reaction speeds must be weighed up. (References???) potential 2 : The potential or electric potential of the nzvi particle depends on the way the particle is produced and will determine the extent to which the particle is drawn to other particles, and hence the extent to which the particles will aggregate. When the potential approaches 0 mv, there is a large chance of aggregation. Particles with a potential greater than +30 mv or smaller than -30 mv are considered stable. The following factors have an influence on the potential: ph: A ph between 8.0 and 8.2 can result in a potential of 0 mv and hence an aggregation of ZVI particles The ionic strength of the solution: As the concentration of cations (e.g. Na +, Ca 2+, Mg 2+, K + ) increases, the potential will approach 0 mv. In laboratory tests usually de-ionised water is used, while the concentrations of mono- and divalent cations in pore water lie between 1-10 mm and 1-2 mm respectively. The potential of classic ZVI particles is -30 ± 3 mv (Zhang and Elliott 2006; Saleh et al. 2008). Research shows that modified ZVI particles with triblock copolymers have the highest potential (- 50 ± 1.2 mv) and are therefore highly mobile in porous media. The high mobility is explained by the electrosteric stabilisation caused by the polymer. 2. ZVI particle-soil particle interaction Aggregation and deposition of ZVI particles onto soil particles can lead to a blockage of the soil pores. The surface modification of ZVI particles (e.g. coating) can considerably increase mobility. 3. Geochemistry 2 The zeta potential or electrokinetic potential stands for the electric potential in the electric double layer around loaded particles. The zeta potential is determined by the difference in electric potential between the dispersion medium and the stationary liquid layer ( slipping plane ) around the particle. The zeta potential is a measure for the electric repulsion between the particles and hence determines the stability of the dispersion. With a low zeta potential the attractive forces will be greater than the repulsive forces. As a result, the dispersion will not be stable and aggregation of the particles will occur. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 22

23 As discussed in the previous paragraphs, the mobility and hence the transport of the ZVI particles is strongly influenced by the geochemical conditions of the aquifer (ionic strength, ph, Eh, DO). The addition of ZVI particles also has an influence on the geochemical conditions of the aquifer: Dissolved oxygen (DO) is consumed quickly Strongly reducing circumstances are created with redox potentials (Eh) below 0 mv ph increases by more than 3 ph units have already been observed in the lab The Eh and DO have a strong influence on the oxidation of ZVI particles and hence on the mobility of the particles (e.g. large quantities of oxygen will oxidise the ZVI particles faster, decreasing mobility). 4. Application method The transport, mixing and injection method must be aimed at preventing contact between the ZVI particles and oxygen or other oxidants as much as possible. The oxidation of the ZVI particles will lead to a reduced reactivity and mobility (Gavaskar et al. 2005; U.S. EPA 2008c). The radius of influence of the injection method can be improved by the following measures: High injection speed in comparison with the natural groundwater flow Creating a dilation and/or cracks in the soil structure via pulsed injection, increased injection pressure or fracturing before injection (pneumatic and hydraulic fracturing) Creating an artificially high hydraulic gradient (connected injection and extraction wells) Surface modifications of MZVI and NZVI particles To increase the reactivity of ZVI particles, aggregation, precipitation, sedimentation and oxidation must be reduced. Coatings can increase the surface load in order to achieve electrostatic or electrosteric stabilisation of the colloidal mixture (see Figure 6). In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 23

24 Figure 5: Possible surface modifications ( coatings ) to stabilise the NZVI particles The mobility of nzvi particles can increase significantly when certain coatings are applied. It has been demonstrated that particles with a coating of polyelectrolytes (e.g. polyacrylic acid) can remain mobile up to 8 months after injection (see Figure 7) (Kim et al. 2009). Figure 7: Transport in a column test of a tracer (upper row), a nzvi solution stabilised with polyacrylic acid (centre) and a nzvi solution without modifications (bottom row). Figure 6: Transport in a column test of a tracer (upper row), a nzvi solution stabilised with polyacrylic acid (centre) and a nzvi solution without modifications (bottom row). Research has shown that coatings can also reduce the interaction between the reactive surface and the geochemical conditions of the reaction medium (He et al., 2007). However, coatings can also lead to a reduction in the reactivity of the nzvi particle via: Inhibition of diffusion and adsorption of the contamination on the reactive surface Reduction in reaction speed by reducing access to the reactive surface Inhibition of diffusion and desorption of the reaction products In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 24

25 4 Known possibilities and limitations of ZVI particles 4.1 Known possibilities ZVIs can be used in different ways for the remediation of groundwater/soil: Given its greater reactivity and the broader range of pollutants which can be treated, micro-scale or nano-scale zero-valent iron can be used as a more efficient material in reactive barriers than the conventional iron particles for the remediation of contaminant plumes. The life of mzvi & nzvi remains a big unknown factor here; Due to the injectability of nzvi & mzvi, in combination with the relative mobility, reactive zones can be created with zero-valent iron. This allows us to, in comparison with reactive barriers, work at greater depths in the subsoil. Even places that are not easy to access (e.g. underneath buildings) can be reached with ZVI if the mobility of the injected ZVI is great enough; Until now, ZVIs were mainly studied and applied for the remediation of source zones and contaminant plumes with high pollutant concentrations. Emulsified nano-metallic particles (EZVI) were developed for the remediation of zones with free-phase chlorinated pollutants (DNAPLs); ZVI particles can also be used for the ex situ treatment of groundwater in a pump-and-treat remediation. This can be done, for instance, in combination with active carbon, in which the groundwater treatment is not only based on sorption processes, but also on transformation processes (Köber et al., 2001); The treatment of other above-ground waterways (tank waste) with ZVI was also found in the literature (Mallouk and Ponder, 2001). 4.2 Known limitations Besides the many possibilities offered by ZVI, there are also a number of limitations involved in the use of these nano- and micro-scale particles. Limited life of nzvi particles: nzvi particles have a larger specific surface area and are therefore more reactive than granular metallic materials (up to factor 3) Besides the reaction with the chlorinated hydrocarbons, nzvi particles just like granular metallic materials also react with water (anaerobic corrosion), but also with a higher reaction rate (Gillham, 2003). Based on anaerobic corrosion rate of zero-valent iron mentioned by Reardon (1995), Gillham (2003) made the following calculation: Granular ZVI (size: 1 mm): estimated corrosion rate 0.2 to 0.6 mmoles/kg Fe0/day, estimated life is 130 years; nzvi (size: 100 nm): 0.4 mmoles/kg Fe0/day, 150 more reactive than granular iron, estimated life 0.8 years; In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 25

26 Decreasing reactivity of (bi)metallic nzvi: It is known that (bi)metallic nano-scale and granular materials are sensitive to a decreasing reactivity which is explained by the deactivation of the catalyst (Muftikan et al; 1996; Gui et al., 2000). Reactivation via flushing of the (bi)metallic materials is possible, but not feasible in situ for ZVIs (Gillham, 2003); Aggregate formation, sedimentation and adsorption: Aggregate formation and sedimentation of ZVI particles lead to a limited spread of ZVI in the subsoil (Schrik et al., 2004). As a result of aggregate formation the specific surface area will also decrease, resulting in a decrease of reactivity (Nurmi et al., 2005). A sufficiently high mobility of ZVIs during the injection is very important, and this is often an obstacle. Vance (2001) states that a thorough site-specific preliminary study is necessary in this respect. This comprises soil flushing (column test) to check whether a lot of colloidal and inorganic dissolved particles elute. This will determine whether the soil (in situ) must be flushed before the injection of NZVI or not. Via batch tests a colloidal suspension must be optimised in order to prevent aggregation of ZVI particles. Feasibility tests are also necessary to determine the exact mobility of the ZVI particles in the aquifer in order to be able to estimate the area of influence of the ZVI injection; Homogeneous distribution in the aquifer: This was mentioned several times as the critical point of the ZVI technology. In case of a homogeneous distribution of ZVIs in the aquifer, not only aggregation, sedimentation and adsorption play a role, but also heterogeneities in the formations. For emulsified ZVIs difficulties were experienced as well to bring the ZVIs into contact with the DNAPLs (Quinn et al., 2005); Cost of ZVIs: The cost of nano-iron is considerable. As the technology is recent, it is hard to estimate the costs. Taking into account that 10 kg of pollutants can be broken down with 100 kg of nano-iron, this technology could be competitive with the more conventional methods (RPM News, winter 2004); In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 26

27 5 Practice: application of ZVI particles for the treatment of soil contamination 5.1 Introduction In the paragraphs below, a short overview is provided of the practical and technical aspects of the application of iron injections. In addition, a practical guide is offered for the remediation design. 5.2 Practical application The U.S. EPA has made a list of 25 sites in the U.S. where nzvi particles are tested or used for the remediation of soil contamination with VOCl. Below, the data from this list have been summarised briefly. In 60% of cases, nzvi is used for the treatment of the dissolved phase. The treatment of source zones requires a large quantity of reactive material, which is why the remediation is not always economically feasible In 56% of cases, the remediation objectives are reached (= usually a clear decreasing trend in pollutant concentrations, leading to a significant reduction in pollutant loads) Observed reduction in pollutant load between 1.5 & 100%, with an average of 70% ZVI particles applied nzvi: 40% BNP: 32% EZVI: 16% Surface-modified nzvi: 8% Other (only catalyst): 4% Slurry composition: average 8 g Fe(0)/l with a range between g Fe(0)/l Application method Gravitational infiltration on permanent filters: 23% Injection on permanent filters: 13% Direct push: 41% Direct push with pneumatic fracturing: 18% Direct push with pressure pulse technology: 4% Observed effects on geochemistry: Decrease in the redox potential to between -100 and -500 mv Decrease in oxygen content ph usually remains stable, slight decrease in some cases Decrease in permeability of the soil is observed due to blockage of the pores In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 27

28 In some cases the natural reductive degradation was stimulated 5.3 Preliminary study Introduction A thorough preliminary study is required in order to determine whether and in what way iron injection can be applied to remediate soil contamination with VOCl. In addition to a thorough prior knowledge of the site to be remediated (geohydrology, geochemistry, detailed contamination situation) it is also advisable to determine the following through lab tests: Injectability (injection proportion, modified ZVI particles, need for stabilisers to prevent aggregation and sedimentation) Mobility of the iron slurry and the radius of influence to be expected Reactivity of the ZVI products (reaction rate, complete degradation possible, matrix demand, etc.) Finally, a field test is highly recommended in order to check the following: Injection method (injection technique, injection pressure) Absorption capacity of the soil (optimal injection volume) Real radius of influence of the injection Effects on the contamination and the geochemistry Based on the information above, it can be studied whether iron injection is feasible as a full-scale remediation technique and can therefore be considered Best Available Technique. The information above must allows to prepare a full-scale remediation (application method, number of injection points, injection distances, number of injection events, amount of product to be injected, results to be expected, cost estimate, etc.) Characterisation of the site to be remediated For the preparation of the remediation design it is essential to carry out thorough characterisation of the site/area to be remediated. The following must be studied in depth: Contamination situation in the area to be remediated Spread of the soil contamination in the vertical and the horizontal plane determination of the soil sections to be treated (= injection sections) Presence of possible pure product (residually or as free phase) determination of amount of ZVI to be injected and/or combination with other remediation techniques Determination of geochemistry Determination of the redox potential and ph determine zero point so that effects of the iron injections can be monitored Determination of the amount of electron acceptors present (oxidants): amount of dissolved oxygen, nitrate, sulphate, Fe(II) and Fe(total) at the different depths in and outside the area to be remediated determination of the amount of ZVI to be injected In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 28

29 Determination of the amount of electron donors present (reductants): organic matter content, TOC, DOC, chemical oxygen demand determination of quantity of ZVI to be injected Geohydrological study Detailed description of soil structure estimate of distribution and radius of influence of ZVI injections, injection volume and duration of injections Groundwater flow speed and direction estimate of radius of influence, design of injection and monitoring geometry Feasibility tests: laboratory tests and pilot tests 1. Laboratory tests The aim of laboratory tests is: To look at the reaction kinetics and the formation of degradation products (complete degradation) study based on batch tests To determine injectability and mobility research using column tests To determine the stability of the iron slurry sedimentation tests To determine the stoichiometry and the amount of reductant necessary per unit of soil volume batch test with several aquifer samples Below, by way of an example, is a description of the laboratory tests which were used for the pilot test with iron injection in Herk-de-Stad (project in the framework of CityChlor): Sedimentation test: A sedimentation test was carried out to check whether the stability of the iron slurry can be improved by adding a substrate (~reduction of particle sedimentation speed). The test was carried out in glass 20 ml vials (vial 1: ZVI + water; vial 2: ZVI + 1:1 water/glycol mixture). The vials were shaken vigorously and subsequently the sediment was monitored visually after 4, 12 & 22 minutes. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 29

30 Sedimentation test with micro-scale iron and addition of glycerol as a stabiliser. Column test: The injectability and the distribution were assessed based on a column test. Previously, area-specific groundwater was abstracted and stored in anaerobic conditions (approx. 5 l) in a nitrogen atmosphere. The groundwater was pumped in an upward stream through two cylinder-shaped glass columns (with a length of 20 cm) at a rate of approx. 5 ml/h. The columns were filled with fine sand with a layer of filter sand at the bottom and at the top to prevent the outflow of fine sand at the entrance and exit of the column. In both columns iron slurry was injected manually. Both before and after the iron injection, at regular intervals, samples were taken from the column influent and effluent. The VOCl concentrations of the samples were determined. Column test with nano-scale and micro-scale iron. Batch test: The reactivity of the different iron types was assessed by means of a batch test. Glass 100 ml bottles were filled anaerobically (in a glove bag filled with nitrogen) with 60 ml anaerobic water to which a saturated TCE solution was added. Previously, 2 ml of nano-scale iron slurry (20%) had been added to one set of bottles, while 2 ml of micro-scale iron slurry (20%) had been added to the other set. A third set acted as neutral set and was carried out without iron. The water phase of each of the bottles was sampled at regular intervals and analysed for VOCl and degradation products. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 30

31 Results of batch test for TCE degradation 2. Pilot test (field test) As practice can differ substantially from the predictions based on laboratory tests, it is always advisable to carry out a field test before proceeding with a possible full-scale remediation. Generally speaking, the results of a pilot test give a more representative view and are more suitable for a large-scale design. The pilot test will also provide important information on the practical conditions for implementation in the site-specific circumstances, namely: The amount of slurry that can be injected and the feasible injection pressure and rate (prevention of break-out at ground level or short-circuit flows. The radius of influence of the injection, and hence the distance required between injection points The pollutant load reduction to be achieved, and possible post-remediation values to be reached The effects on the geochemistry The duration within which the (effects of the) ZVI particles remain active The possible rebound effects Based on the literature studied, usually an injection test is used as a pilot test. During an injection test, a certain volume of iron slurry is injected or infiltrated. The radius of influence is monitored in surrounding probes at specific distances from the injection point. To monitor the radius of influence, boreholes (liners) can be made as well around the injection point, to enable visual inspection of any present iron slurry. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 31

32 5.4 Dosage calculation Based on the results of the characterisation of the site and the lab tests, the reductant consumption can be calculated. The zero-valent iron will not only reduce the contamination but also react with other electron acceptors, such as oxygen, nitrate & sulphate. The table below shows an overview of the electron demand for the main electron acceptors: Electron acceptor Electron equivalent per mol or electron acceptor Required amount of Fe(0) (mol) Oxygen 4 2 Nitrate Sulphate 8 4 PCE 8 4 TCE 6 3 DCE 4 2 VC 2 1 Table 2: Overview of the electron demand for the main electron acceptors If there is natural degradation, one must also take into account other potential electron donors, such as organic matter Per mol of organic carbon 4 mol of electrons can be produced, depending on the oxidation state. Based on the electron balance shown above, the theoretically required amount of Fe(0) per soil volume can be calculated. For the following reasons: Electron demand can never be known 100% due to uncertainties during the characterisation of the site and the lab tests The zero-valent iron also oxidises with water The oxidised iron forms a shell around the iron particle, as a result of which not all the zero-valent iron present is able to react The zero-valent iron is not distributed homogeneously in the soil One must take into account a safety factor between 5 and 10 (see also the case study in Chapter 6). 5.5 Application methods Paragraph 2.5 shows that the application method has an important influence on the distribution of the zerovalent iron. As remediation with zero-valent iron is based on direct contact between the ZVI particles and the contamination, a good distribution of the ZVI particles and hence the choice of the application method are of crucial importance. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 32

33 In the table below an overview is given of the available application methods. Method Description Disadvantages Advantages Injection via filters Injection of iron slurry under pressure via vertical permanent injection filters Cost of placing injection filters. Not very flexible technique. Iron slurry can clog the filter package, which makes reuse impossible or very difficult A second injection round is simple and cheap. With a good grout stop, higher injection pressures are possible than with direct push Direct push Injection of iron slurry via a direct push method Carrying out a second injection round is expensive (mobilisation Flexible technique that allows us to treat specific depths and use of a probe). Injection pressures are limited to prevent breakout of slurry to ground level. Gravitational infiltration Passive (gravitational) infiltration via permanent Can only be applied in highly permeable soils. A second injection round is simple and cheap. filters Iron slurry can clog the filter package, which makes reuse impossible or very difficult. Combined injection and abstraction Combination of direct injection or infiltration and High cost. Any reinfiltration must occur Increase of radius of influence of the injection. simultaneous groundwater abstraction (creation of high hydraulic gradient improved distribution) under anaerobic conditions. Table 3: Overview of the most important application methods The above application methods can be complemented with techniques which may possibly lead to a better distribution of the iron slurry: Pneumatic or hydraulic fracturing: The creation of cracks in not very permeable soil with compressed air (nitrogen to maintain a low-oxygen environment) or pressure pulse via water Pressure pulse technology (PPT): injection using high-frequency pressure pulses, causing the dilation of the soil pores. This technique can also be applied with direct push (using packers) or in permanent filters. Remark: The above techniques can create preferential channels, which make it impossible to determine the distribution and the area of influence of the iron slurry. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 33

34 The injection pressure in porous media is lower than 1-2 bar. The injection pressure must in no case exceed the matrix pressure3 if one wants to avoid vertical spread to the surface. 5.6 Monitoring of the remediation Monitoring during the injections At least the following parameters must be monitored during the injections: Injected volumes and concentrations of iron slurry per injection point and interval Injection pressure (prevention of break-out to ground level) Monitoring after the injection In order to determine the influence of the iron injections on the contamination situation, the following parameters must be measured: VOCl Degradation products ethene, ethane and methane In order to determine the influence of the iron injections on the local geochemistry, the following parameters must be monitored: Measurement of Fe(II) and Fe (total) contents. These measurements are important to determine the radius of influence of the zero-valent iron injection. The radius of influence can be deduced from significant increases in the Fe(II) and Fe(total) content before and after the injection and after oxidation of the zero-valent iron. Measurement of local redox conditions: Redox potential measurements Oxygen content Nitrate and sulphate The contents measured must be compared to those found before the injections. In order to determine the inflow of these parameters, a probe located upstream must also be sampled. In order to study the spread of organic substrate that may have been added (e.g. stabiliser of the ZVI particles or emulsified iron), the DOC content (dissolved organic carbon) must be monitored in the surrounding probes. The evolution of the DOC content can also tell something about the presence or stimulation of natural degradation. 3 Matrix pressure or effective stress: σ e = σ s P where σ s is particle stress (σ s = ρ s x g x h) and P water pressure (P = ρ w x g x h). In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 34

35 5.7 Evaluation diagram for use of iron injection The decision framework for the application of iron injections has been added under appendix Stability/safety ZVI particles, and especially nzvi particles, are not stable. In dry form, the powder ignites immediately when it enters into contact with air. Storage in dry form can only take place in an inert atmosphere. For this reason, nzvi is usually supplied as a slurry, but even so the product can only be stored for a limited amount of time, as it reacts with water. Contact between nzvi and important oxidants such as oxygen, sulphate, nitrate and water must be avoided. The life time also depends on the ph of the suspension and the particle type. A low ph causes an accelerated oxidation of the nzvi particles (Lowry, 2005). Open flame near the ZVI particles is strictly prohibited as nzvi particles can generate hydrogen (reaction with water). There is a warning about dangerous reactions when the product is mixed with oxidising agents (exothermic reaction) or acids (production of hydrogen). It is recommended to store the product in a cool place and prevent drying. ZVI particles are described as a product that is not hazardous to man or nature Protection of the skin (PVCcoated gloves, protective clothing), eyes (safety goggles) and airways (usually not necessary) is recommended. Any spilled product must be cleaned with a large amount of water as soon as possible. All chemicals that are used for the purposes of ISCR must fulfil European Union REACH (registration, evaluation, authorisation & restriction of chemicals) guidelines. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 35

36 6 Cost 6.1 Cost of ZVI particles The price of nzvi particles varies between 25 and 325 euro/kg Fe(0). This variation in price can be put down to the manufacture and also the type of nzvi (stabilised products, modified products, conservation). Micro-scale and granular Fe(0) are available for less than 1 euro/kg (see also paragraph 3.4, excluding costs for transport and handling). 6.2 Cost of preparing full-scale iron injection The preparation for a full-scale remediation typically includes all the necessary lab tests, followed by a field test. On the basis of the experiences with the CityChlor pilot 4 in Herk-de-Stad, the costs of the entire pilot can be divided up as follows (including consultancy costs, rounded up to significant figures, excluding VAT): Post Cost (keuro, excl. VAT) Share (%) 1. Lab tests Field test - Injections Cost of ZVI material Monitoring and interpretation Total % Table 4: Division of costs CityChlor pilot Herk-de-Stad 4 Voor meer details met betrekking tot deze pilootproef wordt verwezen naar het praktijkvoorbeeld onder bijlage 1. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 36

37 The following costs were incurred during the pilot for ZVI materials: ZVI material Form Cost price ZVI material nzvi: NANOFER 25 Nano Iron mzvi: Gotthart Maier Metallpulver GmbH Slurry (20% dm) Powder 25 Euro/kg slurry = 125 Euro/kg ZVI dm 1.2 Euro/kg ZVI dm Price of ZVI per m injection phase (Euro/m) (*) Price of slurry per m injection phase (Euro/m) 62,5 62, (**) 260 Price of slurry ZVI/m³ soil (Euro/m³ soil) (*) 50 l of slurry per metre were injected with 50 g mzvi/l and 10 g nzvi/l injected (**) The mzvi was dissolved in a 40% v/v solution with glycerol. Glycerol costs approximately 1.3 euro/l. (***) Based on the volume of soil that comes into direct contact with the injected solution (theoretical radius of influence of 0.28 m around each injection point. Table 5: Costs of ZVI materials CityChlor pilot Herk-de-Stad 6.3 Cost of full-scale remediation iron injection 1. General estimate of cost of remediation per m³ of treated soil On the basis of the full-scale remediation in Rotterdam (practical case Wegrosan hmvt, see appendix 1) and 5 practical cases 5 that were found in the literature, which provided sufficient financial information, the remediation costs varies between (rounded to significant figures) 20 and 370 euro/m³ of treated soil 6 (for remediation of polluted soil volumes ranging from 735 to 24,000 m³). The cost per volume of treated soil is inversely proportional to the scale of the project. 2. Summary of cost division for full-scale remediation On the basis of 3 practical examples from the literature and the practical example from Rotterdam (Wegrosan hmvt, see appendix 1) of full-scale remediations, for which there is sufficient financial information, the total price can be categorised (in order of significance) as follows: 5 Mueller, N. C. and Nowack, B Nano zero valent iron - THE solution for water and soil remediation? Report of the ObservatoryNANO at U.S. EPA. (2008) Nanotechnology for Site Remediation Fact Sheet Prijzen cases uit de literatuur zijn afkomstig van amerikaanse en europese studies uitgevoerd in de periode De dollarprijzen werden omgerekend naar euro op basis van een wisselkoers van 1,3 dollar / euro. In de studies wordt de kostprijs meestal zeer summier vermeld, de opdeling van de kosten is eveneens niet voor elke studie dezelfde. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 37

39 7 Results of market survey In order to supplement this study, a market survey was conducted to gauge experiences with ISCR according to the following parties: Soil remediation companies (cases and/or specific experiences with zero-valent iron): European Remediation Technologies (ERT) DEC Ecoterres Groundwater technology In-Situ Techniques Mourik Verhoeve Milieu Wegrosan hmvt Suppliers of zero-valent micro and nano-scale iron: FMC Technologies (EHC, EHC-L, Daramend ) Regenesis (CRS ) The market survey was conducted on the basis of a questionnaire (see appendix 2) which initially gauged the experiences (# cases) of the soil remediation company/supplier with respect to the application of iron injections for the treatment of soil contamination with chlorinated components ((ISCR). The second part of the survey asked for further details about issues such as the pollution situation, soil texture, injection method, type of iron product (and supplier), cost and any points of particular attention. On the basis of the surveys returned, a number of contactors were selected for an in-depth interview. On the basis of this interview, a final decision was made regarding which practical cases would be added to the study at hand (see chapter 9). The following information was obtained on the basis of the surveys: Soil remediation companies (cases and/or specific experiences with zero-valent iron): European Remediation Technologies (ERT) no experience with ISCR DEC Ecoterres no experience with ISCR Groundwater technology no response to survey In-Situ Techniques no experience with ISCR Mourik no experience with ISCR Verhoeve Milieu experience with ISCR (2 cases) Wegrosan hmvt experience with ISCR (5 cases) Suppliers of zero-valent micro and nano-scale iron: FMC Technologies (EHC, EHC-L, Daramend ) experience with ISCR (1 case) Regenesis (CRS ) no info on ISCR practical examples On the basis of the information supplied, in consultation with OVAM, a decision was made to interview Verhoeve Milieu and Wegrosan hmvt. The following cases were then discussed: Verhoeve Milieu Case 1: CityChlor Herk-de-Stad, Belgium - Pilot: Chemical reduction using nzvi and mzvi In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 39

40 This case is fully elaborated and added under appendix 1. - Case 2: Project in Huizingen, Belgium Pilot: Injection of nzvi dissolved in organic substrate PV1 for the treatment of soil contamination with trichloroethane Before the remediation of soil contamination with TCA, a number of injections were carried out with nzvi dissolved in the organic substrate PV1 (mixture of alcohols and sugars). Prior to the injections, no lab tests were carried out, the injection doses were set on the basis of the estimated contamination loading (see paragraph 4.4) with a safety factor of 10. The injections were carried out via a direct push in the form of a hexagonal template (centre-to-centre distance of injections = 4 m). After injection, a redox potential of -200 mv was recorded at the site. According to Verhoeve Milieu, the low redox potential can be put down to the injections of the substrate. Eighty days after injection, a reduction in the contamination loading of 80% was observed (1000 µg/l TCA (200 µg/l TCA). Assessment of iron injections by Verhoeve Milieu: ZVI is practical as a management tool in permeable reactive barriers. As a result of the limited spread of the ZVI particles, the use of iron injections as an active remediation measure is not recommended. Wegrosan hmvt Case 3: Rotterdam, the Netherlands Full-scale remediation: Stimulation of anaerobic organic decomposition and chemical reduction using EZVI. This case has been fully elaborated and added under appendix 1. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 40

41 8 Conclusions The remediation of soil contamination by means of iron injections is based on direct contact between the zero-valent iron and the contamination. Good injectability, stability and mobility of the iron particles are crucial for this. Based on our own practical experience (pilot CityChlor Herk-de-Stad) and the literature studied, these are the main bottlenecks of iron injection as a remediation technique. In comparison with other, more conventional remediation techniques, there is a high remediation cost involved in the use of iron injections, as this technique requires a very dense network of injection points and/or the application of advanced injection techniques in combination with expensive modified iron particles. The use of iron injection (with EZVI) for the treatment of areas with DNAPL is possible in theory, but is not considered cost-effective, as it requires a large amount of (expensive) zero-valent iron and a good contact between the DNAPL and the zero-valent iron cannot be guaranteed. Based on our own practical experience and the study at hand, iron injection as an independent remediation technique is less suitable for the remediation of soil contamination with VOCl. The use of iron injections, however, is deemed to be useful for creating suitable geochemical circumstances for the natural reductive breakdown of VOCI contamination (see also the practical examples under appendix 1). In other words, zero-valent iron can serve as an aid for stimulating natural decomposition: a combined injection of substrate and zero-valent iron (e.g. emulsified zero-valent iron) in (source) areas with unsuitable geochemical conditions or strongly increased (~toxic for biology) concentrations of contaminants. On the basis of our own practical experiences (pilot CityChlor Herk-de-Stad) and the literature review, the increased efficiency of the remediation and the reduced remediation costs depend on the radius of influence of the injection. Additional research into expanding the radius of influence of iron injections is thus vital. Expanding the radius of influence can be achieved via 1. Improved injection techniques 2. Modified zero-valent iron A tried-and-tested and effective use of zero-valent iron (granular) involves PRB (permeable reactive barrier) for the treatment and management of the retardation zone. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 41

46 10 Appendix 10.1 CityChlor Herk-de-Stad: Trial Test ISCR injection nzvi & mzvi Introduction On commission from OVAM, the temporary partnership Tauw België NV Verhoeve Group Belgium conducted during the period June 2011 January 2013 a trial test on In-Situ Chemical Reduction (ISCR) of chlorinated solvents (VOCl). The trial test is part of the CityChlor project. More concretely, in the vicinity of a source zone contaminated with VOCl, located on the site of a former printing plant in Herk-de-Stad, soil injections were carried out with micro and nano-scale zero valent iron (MZVI & NZVI). The trial test was conducted in 4 stages: Stage 1: Eco-technical evaluation of the site interpretation of earlier investigation findings and characteristics of the injection zone Stage 2: Design and performance of lab testing Stage 3: Injection of iron particles Stage 4: Monitoring geochemical and eco-technical parameters following injection Conceptual model Geo-hydrology Based on the performed characterisation core drillings, the soil composition strata may be summarized as follows: m down from ground level: silty to clayish moderately coarse-grained sand m down from ground level: presence of thin layer of peat (* not observed in all drillings) m down from ground level: silty to clayish moderately coarse-grained sand m down from ground level: sandy thin layer of peat (*not observed in all drillings) m down from ground level: moderately coarse-grained sand >14.5 m down from ground level: clay The phreatic groundwater lies at ca. 3 m down from ground level The horizontal groundwater flow direction may, albeit not entirely uniformly, be considered to be north-westerly. On the basis of the current horizontal level reference, a very limited hydraulic gradient (ca. 0.2 %) is calculated. The following table shows the hydraulic conductivity per depth-trajectory: In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 46

47 Table 10.1 Hydraulic conductivity Soil layer Hydraulic conductivity 0-5 m down from ground level 1.67 x 10-6 m/ s or 0.14 m/d 5-10 m down from ground level 3.2 x 10-6 m/s or 0.28 m/ m down from ground level 2.64 x 10-7 m/s or 0.02 m/d Taking into account the highest observed average hydraulic conductivity and a 15% porosity, the groundwater s maximum flow velocity is 1.3 m / year or 0.35 cm / day at a calculated gradient of 0.2%. Operating procedures: Determination of the soil composition strata: passive soil sampling sonic drilling with aqualock sampling. grading curves Groundwater flow direction: water level Groundwater flow velocity: permeability tests (recovery tests depth levels), theoretical permeability on the basis of grading curves Determination of clay and organic content ( depth levels) Contamination condition At and near the product storage site of the former printing plant, major quantities of solvents have seeped into the soil. The parent product is tetrachloroethene (PER). Prior to the start of the trial test, we had no indications of natural reduction breakdown of the contamination with tetrachloroethene (no formation of the breakdown products cis-dichloroethene and vinyl chloride, ethene, ethane). In the solid part of the soil we observed heightened concentrations (>BSN) of tetrachloroethene particularly in the top stratum (0-5 m down from ground level). In the examined groundwater stratum (3-15 m down from ground level) we observed sharply increased concentrations of tetrachloroethene (>BSN). The highest VOCl concentrations are generally found in the top of the groundwater stratum (3-5 m down from ground level). No DNAPL has been observed, but the highest noted concentration of PER ( µg/l ~ 50% maximum solubility, pb1012) points to the possible presence of pure or residual product. In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 47

48 Figure 1: Cross-section of the conceptual site-model West East Injection Injection Groundwater flow Silty clayish mg sand Operating procedures: Updating contamination condition in the vicinity of the suspected core zone Analysis of soil samples ( depth levels) Analysis of groundwater at 3 depth levels MIP-probes DNAPL investigation Control on soil samples by means of the Sudan IV lysochrome Measuring the sinking layer with a meter and teflon bailer Geochemical and redox conditions Based on the performed investigations, the shallow, highest contaminated groundwater stratum does not present the right conditions to promote the natural reduction breakdown of tetrachloroethene (too high an O 2-content, too high a redox potential). In the deeper groundwater stratum, the conditions for natural reduction breakdown of tetrachloroethene are theoretically feasible. On the basis of the measured breakdown and geochemical parameters, we have not found evidence of natural breakdown (no formation of the breakdown products cis-dichloroethene, vinyl chloroethene, ethene, ethane, and no electron-acceptor reduction). Operating procedures: Determination of geochemical and redox conditions Determination of the redox potential ( depth levels) Determination of the oxygen, nitrate, sulphate, Fe(II) and Fe(total) content ( depth levels, within and outside the core zone) In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 48

50 Injectability and distribution of mzvi & nzvi Tube test Injection of a volume of nzvi- & mzvi-slurry in a tube containing aquifer material. Subsequently, the tube is subjected to a constant flowthrough in order that the distribution can be monitored (visually). Before injection After injection Nano-iron micro-iron Photo 2 Tube test before and after injection of the iron slurry The tube test shows that the iron is primarily distributed in reaction to the injected volume (~filling the void volume); following the injection, one may consider the iron particles to be immobile. On the basis of visual observations, it appears that most of the iron mass migrated in the tube only up to 2 to 3 cm. The nano-scale iron demonstrated better distribution than the micro-scale iron (no visual build-up at the point of injection point). Nonetheless, further migration under the influence of the groundwater flowthrough likewise appeared limited. Reactivity Batch test The reactivity of both iron types was also tested out by means of a batch test. Seeing that the breakdown reactions happen at the surface of the iron particle, it is expected that the nano-scale iron, having a greater specific surface area, can generate higher breakdown velocities. TCE Concentration Time (days) blank / nano-iron / micro-iron In Situ Chemical Reduction using Zero Valent Iron injection - A technique for the remediation of source zones 50

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